Ink printing apparatuses can be used for single-color or multicolor printing to a printing substrate web (of a belt-shaped recording medium, for example) made of the most varied materials (paper, for example). The design of such ink printing apparatuses is known; see EP 0 788 882 B1. Ink printing apparatuses that operate according to the Drop on Demand (DoD) principle have a print head or multiple print heads with nozzles comprising ink channels, the activators of which nozzles—controlled by a printer controller—excite ink droplets in the direction towards the printing substrate web, which ink droplets are directed onto the printing substrate web in order to apply print dots there for a print image. The activators can generate ink droplets thermally (bubble jet) or piezoelectrically.
In the document DE 100 27 471 it is described that a change of the tension of the print medium—in particular transverse to the transport direction—is monitored and corrected via measurement and comparison of the circumferential velocities of the transport rollers (thus during the printing process).
Upon printing to a printing substrate web, it is sometimes necessary to hold the printing substrate web in a pause function during the printing operation, for example in order to monitor the register quality after printing a print job, or in order to remedy problems in the post-processing of the printing substrate web. After running the printing substrate back up, print image disruptions can then occur at those web segments that were located directly beneath the print heads after activation of the pause function. Due to the relatively large transfer printing zone in ink printing apparatuses (for example inkjet printing systems), in particular given color printing, the print image disruptions that are created due to the pause correspond to a great deal of spoilage. The occurring print image defects include print image distortions and color registration errors. The causes of these are the swelling or shrinking of the printing substrate web during the pause and—connected with these—the position shifts of the printing substrate web below the print heads (in particular in the longitudinal direction) since the printing substrate web below the print heads continues to remain under tension.
These problems are explained using FIG. 1. Of a printing apparatus DR, a printing unit 1 and a printer controller 2 are shown. The printing unit 1 is arranged along a printing substrate web 3, which printing unit 1 has the print bar 4 with print heads 5 in series as viewed in the transport direction PF0 of the printing substrate web 3. Given color printing, for example, a respective print bar 4 can be provided per color to be printed. The printing substrate web 3 is moved past the print bars 4 with the aid of a take-up roller 9; it thereby rests on a saddle with guide rollers 8. A rotary encoder or encoder roller 6 is arranged at the intake of the printing unit 1, which rotary encoder or encoder roller 6 is driven by the printing substrate web 3 and—depending on the feed movement of the printing substrate web 3—generates counting pulses that are supplied to the printer controller 2 and are used by the printing controller 2 to (for example) establish the point in time of the triggering of the printing process at the individual print heads 5. The printing substrate web 3 is supplied to the encoder roller 6 by a drive roller 7 arranged before the encoder roller 6.
In FIG. 1 it is now presented in principle how the printing substrate web 3 can be affected by the printing unit 1 or the environment air in individual web segments BA through the printing apparatus DR, for example given a standstill of the printing apparatus DR. The printing substrate web 3 is exposed to the environment air in the web segment BA1 between drive roller 7 and encoder roller 6, with the consequence that here a swelling of the printing substrate web 3 can occur due to the moisture of the environment air. The change of the printing substrate web 3 in the longitudinal direction that is due to this is, however, compensated with the aid of the encoder roller 6. In the web segment BA2 after the encoder roller 6, up to the printing unit 1, a swelling of the printing substrate web 3 due to the environment air can occur that, however, remains unconsidered by the encoder roller 6. This also applies for the web segment BA3 under the print heads 5 of the printing unit 1. In this transfer printing zone BA3, the printing substrate web 3 can shrink due to the operating temperature of the print heads 5. However, the printing substrate web 5 is also exposed to the environment air, such that the web segment BA3 can swell due to the moisture in the environment air, in particular given greater distances between the print bars 4. The two influences overlap. The printing substrate web 3 is thus exposed to different environment influences from the drive roller 7 up to the take-up roller 9, which environment influences can lead to either a shrinking or swelling of the printing substrate web 3. Since the printing substrate web 3 is furthermore directed under tension past the print bars 4, a displacement of the printing substrate web 3 under the print bars 4 results due to the length changes of the printing substrate web 3 that are caused by environment influences. This can lead to the print image errors mentioned above if the printing process is started again after an interruption of the printing operation.
The Application DE 10 2011 054 693 A1 (corresponding to US 2013/0100194 A1) describes a correction of the tension of the print medium during a printing pause to compensate for swelling or shrinking of the medium. For this, a web tension sensor in proximity to a rotary encoder wheel is used